1. Field of the Invention
The present invention relates to a laser oscillation device of the discharge excitation type employing a gas as the laser medium, that is used in for example laser processing, medical treatment, illumination or communication and in particular relates to a gas laser oscillation device having a function of controlling the transverse mode of the laser output (hereinbelow referred to as “beam mode” or simply as “mode”).
2. Description of the Related Art
Gas laser oscillation devices are widely employed in for example laser processing, medical treatment, illumination or communication and gas laser oscillation devices are known having a plurality of discharge sections, respective power sources for discharge excitation being connected to electrodes provided in each discharge section. FIG. 8 is a view showing the layout of major parts of a typical example thereof. In this Figure, reference symbols 4a and 4b represent respectively a rear mirror that does not have partial transparency, and an output mirror that has partial transparency. An optical resonance space is formed between these two mirrors 4a and 4b. In this optical resonance space, there are provided two discharge sections 3a and 3b. 
The discharge sections 3a and 3b respectively comprise electrodes 22a and 22b. The electrode 22a is connected with a power source 1a for discharge excitation and the electrode 22b is connected with a power source 1b for discharge excitation. The discharge sections 3a and 3b are of the same size and the same shape and, likewise, for the electrodes 22a and 22b, electrodes of the same size and same shape and same electrode construction are employed.
The power sources 1a, 1b for discharge excitation are of the commonly known type that can be respectively independently operated and wherein the power that is supplied to the discharge sections 3a, 3b can be freely adjusted.
The laser medium gas is circulated through a circulatory path passing through the optical resonator using a fan 6. The medium gas that is fed from the fan is supplied to the discharge sections 3a, 3b after passing through a heat exchanger 5a to remove the heat of compression. The laser medium gas is excited by discharge in the discharge sections 3a, 3b to generate laser light. The laser light that is thus generated is amplified within the optical resonator by well-known principles and the output laser beam is extracted from the output mirror 4b. After emitting the laser light, the gas medium, which has reached a high temperature due to the discharge, is cooled in a heat exchanger 5b and again returned to the fan 6. In this example, the two discharge sections 3a and 3b are constituted by two discharge tubes and are driven by the two power sources 1a, 1b for discharge excitation, which are operated independently. The power sources 1a, 1b for discharge excitation are AC power sources, so the discharge that is created in the discharge sections 3a, 3b is an AC discharge.
Typically, in a gas laser oscillation device of this type, a beam mode is formed that is determined by the construction and dimensions of the optical resonator. That is, various different beam modes can be formed, depending on the length of the optical resonator (length of the optical path between the mirrors 4a and 4b) and the cross-sectional shape and dimensions or the like of the discharge sections. Also, when the discharge section is constituted by a discharge tube, the beam mode that is formed is determined by its internal diameter, the shape of the electrode and, in addition, although not shown in the drawing, the internal diameter of the aperture that is arranged on the optical path. Disclosure concerning this is made in for example Japanese Patent Application Laid-open No. 64-42187.
In fact, what sort of beam mode should be produced when employing the laser oscillation device is determined by the object, such as processing, for which this device is to be employed. However, in order to be able to cope with a wide range of applications, it is desirable that this beam mode should be capable of being suitably controlled to exhibit characteristics matching the application.
Regarding the control of this beam mode, a method of changing the mode that is widely employed conventionally is to introduce into or remove from the optical path an aperture, as disclosed in European Patent Application Laid-open No. 0492340.
Specifically, mode changeover is effected between the TEM00 mode (gaussian mode) or low-order mode and TEM01* (ring mode) or higher-order modes when an aperture is arranged on the optical axis or when the aperture is removed from the optical axis. However, typically, methods of mechanically moving such an aperture involve problems relating to durability or response, high-speed control thereof is difficult and adjustment of the optical axis of the aperture is also difficult, and such methods are very costly.
A further example of disclosure of prior art is Japanese Patent Application Laid-open No. 2002-118312. This relates to improvements in a method of controlling beam mode. In the embodiment of this publication, an adaptive mirror (mirror whose curvature can be changed) is employed for mode control and a technique is illustrated of changing over the mode by setting two mechanically changeable mirror curvatures. However, even with this method in which mirror curvature is changed, problems relating to response, controllability, difficulty of optical axis adjustment and cost still arise. A detailed description of the function and construction of an adaptive mirror is to be found in Japanese Patent Number 3072519.